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Oklahoma has had more earthquakes over magnitude 3 in 2014 than California.

“Seismic swarms” of earthquake activity have been recorded in Oklahoma, according to just-released research led by Dr. Katie Keranen at the Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York.

Oklahoma earthquakes in areas of high-rate water disposal constitute nearly half of all central and eastern US seismicity from 2008 to 2013.

Unconventional oil and gas production provides a rapidly growing energy source; however high production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes.

In the research findings, reported in SCIENCE, Dr. Keranen writes, “Subsurface pressure data required to unequivocally link earthquakes to injection are rarely accessible. Here we use seismicity and hydrogeological models to show that distant fluid migration from high-rate disposal wells in Oklahoma is likely responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper-basement, between 2-5 km depth.

Seismicity in the United States midcontinent surged beginning in 2008, predominantly within regions of active unconventional hydrocarbon production. In Arkansas, Texas, Ohio, and near Prague, Oklahoma, recent earthquakes have been linked to wastewater injection although alternative interpretations have been proposed, writes Dr. Keranen.

Sharp increase in Oklahoma magnitude 3+ earthquakes.

Conclusively distinguishing human-induced earthquakes based solely on seismological data remains challenging. Seismic swarms within Oklahoma dominate the recent seismicity in the central and eastern United States, contributing 45% of M3 and larger earthquakes between 2008-2013. No other state contributed more than 11%.

Our work demonstrates that a very small number of wastewater disposal wells, operating at exceedingly high volumes, create substantial anthropogenic seismic hazard, Dr. Keranen writes. Four of the highest-volume disposal wells in Oklahoma (~0.04% of wells) are capable of triggering ~20% of recent central US earthquakes in a swarm covering nearly 2000 square kilometers, as shown by our analysis of modeled pore pressure increase at relocated earthquake hypocenters. This massive Jones seismic swarm is in sharp contrast to the oft-cited aseismic behavior of the majority of active disposal wells. The area of increased pressure related to these wells continually expands, increasing the probability of encountering a larger fault and thus increasing the risk of triggering a higher-magnitude earthquake.

These results from Oklahoma, where the rate of M3+ earthquakes is now double that of California, are broadly relevant for understanding fluid-related earthquake triggering and rapid fluid transmission in the shallow crust to great distances. Our results illuminate the sharp response of faults in critical equilibrium to the strong forcing mechanism of massive subsurface fluid injection.

With growing development of unconventional reservoirs, the number of new disposal wells drilled and the volume of water disposed at individual wells have abruptly increased. The burgeoning hydrocarbon production from unconventional reservoirs has sharply changed energy and energy politics, and states from California to New York are currently debating permitting and regulation of hydraulic fracturing and wastewater disposal.